Currently, phpBB always encodes the following headers in Base64:
It is appropriate to encode these in Base64 if they include characters that are not contained in the ASCII character set. However, if these fields only contain ASCII characters they should not be Base64 encoded. Encoding them as Base64 makes them non-human readable when looking at the raw email source. It is also a common obfuscation tactic in spam, which is why Spamassassin tests for these with the FROM_EXCESS_BASE64 test (does not apply to the "To" and "Subject" fields).
It should be fairly simple to have phpBB check for non-ASCII characters before encoding these headers in Base64.